The present invention relates to a lithographic printing plate precursor which requires no development and is excellent in press life. More specifically, the present invention relates to a lithographic printing plate precursor capable of plate-making by heat mode image-recording, also capable of image-recording by scanning exposure based on digital signals, and capable of mounting on a printing machine for plate-making and printing with requiring no development.
A lithographic printing plate generally comprises a lipophilic image area which receives an ink and a hydrophilic non-image area which receives fountain solution during printing. As such a lithographic printing plate precursor, a PS plate (presensitized plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer has so far been widely used.
On the other hand, digitized techniques of electronically processing, accumulating and outputting image data by using a computer have prevailed, and various image output systems corresponding to these digitized techniques have been put to practical use. As one example of such techniques, a computer-to-plate technique directly making a printing plate is attracting public attention, which comprises a step of scanning exposing a printing plate precursor with high convergent radiant rays such as laser beams carrying digitized image data without using a lith film. With such a tendency, it has become an important technical subject to obtain the printing plate precursor well adapted to this purpose.
Solid state lasers having high output, e.g., a semiconductor laser and a YAG laser are inexpensively available in recent years. As a result, as a producing method of a printing plate by scanning exposure which is easy to be incorporated in a digitized technique, a plate-making method using these lasers as an image-recording means is promising.
In conventional plate-making methods, image-recording is performed by imagewise exposing a photosensitive printing plate precursor in low to middle intensity to cause the change of imagewise physical properties on the surface of the precursor by a photochemical reaction. On the other hand, in a method of using the exposure of high power density by a high output laser, a large quantity of light energy is irradiated on an exposure region convergently during a momentary exposure time, the light energy is efficiently converted to heat energy to cause a chemical change, a phase change, or a thermal change such as the change of form and structure due to the heat, and that change is utilized in image-recording. That is, image data are inputted by light energy such as laser beams, but image-recording is performed by the reaction due to heat energy. This recording system making use of heat generation by high power density exposure is generally called heat mode recording and converting light energy to heat energy is called light-to-heat conversion.
A big advantage of a plate-making method utilizing a heat mode recording means is that a material is not sensitive to light of general intensity level, such as room illumination, and the image recorded by high intensity exposure does not necessitate fixation. That is, when a heat mode material is used in image-recording, the material is safe to room light before exposure and fixation of the image after exposure is not essential.
Accordingly, if heat mode recording is utilized, it is expected that it will be possible to obtain a lithographic printing plate precursor which is easily developed to a computer-to-plate system.
As one preferred plate-making method of lithographic printing plate on the basis of heat mode recording, a method has been suggested which comprises the steps of providing a hydrophobic image-recording layer on a hydrophilic substrate, imagewise exposing the hydrophobic layer by heat mode exposure to change the solubility and dispersibility of the hydrophobic layer, and, if necessary, removing the non-image area by wet development.
As an example of such a printing plate precursor, there is disclosed in JP-B-46-27919 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) a method for obtaining a printing plate by heat mode recording a printing plate precursor comprising a hydrophilic support having provided thereon a recording layer showing a so-called positive function, i.e., a recording layer having a function whose solubility is improved by heat, specifically a recording layer having a specific composition such as saccharides and melamine-formaldehyde resins. Since the disclosed simple plate-making techniques of heat mode recording including the above method are in general not sufficient in heat sensitivity, the sensitivity is extremely insufficient for heat mode scanning exposure. Hence the discrimination of hydrophobicity/hydrophilicity of the irradiated area and the non-irradiated area, i.e., the discrimination of the image area and the non-image area, is small, which has been the restriction in practical use. If the discrimination is insufficient, plate-making according to the on-press development system is substantially difficult.
As the means to solve that problem, methods to remove the image layer at the irradiated area by heat splashing due to the work of heat by high output laser beam irradiation (called abrasion) are disclosed, e.g., in WO 98/40212, WO 98/34796 and JP-A-6-199064 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), specifically lithographic printing plate precursors capable of plate-making without performing development which comprises a substrate having thereon plural layers comprising a hydrophilic layer containing transition metallic oxide colloid as the upper layer and a lipophilic image-recording layer as the lower layer are disclosed. The discriminability of the irradiated area and the non-irradiated area where heat splashing has been completely performed is certainly large according to these methods, but there arise other problems that the printing machine (i.e., the printing press) is stained by the splashed matter, the stain on the printing plate surface impairs the operation of the printer and printing quality, in addition, the heat of the irradiated light often does not reach the deep part of the image-recording layer, as a result, the bottom part of the image-recording layer close to the support is not splashed and remains, i.e., the phenomenon called a residual film is brought about. Substantial discriminability cannot be exhibited due to the residual film, which leads to the reduction of printing quality.
As is the situation, as a method not accompanied by such drawbacks, there are disclosed simple plate-making methods making use of the change of the degree of hydrophilicity/hydrophobicity of the surface by heat, i.e., the change of polarity, not according to abrasion even when an image is formed by heat mode light irradiation. For example, methods comprising the steps of the addition of a thermoplastic polymer such as hydrophobic wax and polymer latex to a hydrophilic layer and hydrophobitization by phase separation to the surface by heat are disclosed in JP-B-44-22957, JP-A-58-199153 and U.S. Pat. No. 3,168,864. These techniques suggest a direction of the improving means of discriminability. However, since these disclosed techniques are insufficient in discriminability and there is apprehension about staining of printed matter due to, in particular, insufficient hydrophilicity, the improvement is desired.
Sufficient discrimination of an image area and a non-image area is a fundamental important characteristic directly linked with the improvement of printing quality, such as printing stain prevention and inking property, and press life, accordingly, the development of a plate-making method having high discriminating property and easiness of print-making operation, in particular, a plate-making method having high discriminating property and high sensitivity, requiring no development process, capable of heat mode plate-making, and excellent in press life and inking property at printing is desired.
Further, according to the study by the present inventors, in the case of image-forming by utilizing heat mode light irradiation, in particular, image formation by utilizing the change of polarity by irradiation of laser beams, a hydrophobic layer is not sufficiently formed if heat diffusion to a support is fast. If the irradiated light amount is increased to avoid the insufficient layer formation, there arises an undesirable problem that the printing machine and the printing plate are stained by heat splashing of the image layer. Therefore, it is desired to bring it into realization to improve discriminating property, to widen a degree of latitude in light irradiation, and at the same time to improve press life and inking property by making it possible to form an image with little energy.
An object of the present invention is to solve the above-described defects of the heat mode plate-making method and improve the performance of the heat mode plate-making method.
That is, an object of the present invention is to provide a heat mode type lithographic printing plate precursor requiring no development process, having high sensitivity and broad latitude in exposure amount, capable of easily plate-making, capable of directly mounting on a printing machine for plate-making, and preventing printing stain of the printing plate surface.
In particular, an object of the present invention is to provide a heat mode type lithographic printing plate precursor from which a plate-making can be easily carried by scanning system image exposure by laser beams, excellent in the discriminability of an image area and a non-image area, having broad latitude in image exposure amount, and having a press life of long duration and excellent in inking property at printing.
As a result of the investigation of the above problems, the present inventors have found that by the provision of a light-to-heat convertible exothermic layer as the lower layer of a hydrophilic photosensitive layer containing metallic fine particles as a light-to-heat converting agent, the discriminability of an image area and a non-image area can be improved, printing staining and press life can be improved, in addition to these, the exothermic amount per a light irradiation amount increases, thus it becomes possible to form an image with less exposure energy, and further, the sensitivity and the latitude can be improved. The present inventors have continued further investigations on the basis of the above knowledge, thus the present invention has been accomplished.
That is, the present invention has been achieved by the following means.
(1) A lithographic printing plate precursor which comprises a support having provided thereon a layer containing a light-to-heat converting agent as a lower layer and a hydrophilic photosensitive layer containing light-to-heat convertible metallic fine particles which change to hydrophobic with the conversion of light to heat as an upper layer.
(2) The lithographic printing plate precursor as described in the above item (1), wherein the metallic fine particles contained in a hydrophilic photosensitive layer are single or alloy metallic fine particles selected from the metallic elements belonging to group VIII or group I-B of the Periodic Table.
(3) The lithographic printing plate precursor as described in the above item (1) or (2), wherein an organic sulfur compound having at least one hydrophilic group and at least one metal-adsorbing group is adsorbed onto the surfaces of the metallic fine particles contained in a hydrophilic photosensitive layer.
(4) The lithographic printing plate precursor as described in the above item (1), (2) or (3), wherein a water-soluble protective layer is further provided on the hydrophilic photosensitive layer.
(5) The lithographic printing plate precursor as described in the above item (1), wherein said hydrophilic photosensitive layer further contains a hydrophobitization precursor having a hydrophilic surface.
(6) The lithographic printing plate precursor as described in the above item (5), wherein the metallic fine particles contained in the hydrophilic photosensitive layer are single or alloy metallic fine particles selected from the metallic elements belonging to group VIII or group I-B of the Periodic Table.
(7) The lithographic printing plate precursor as described in the above item (5) or (6), wherein an organic sulfur compound having at least one hydrophilic group and at least one metal-adsorbing group is adsorbed onto the surfaces of the metallic fine particles contained in the hydrophilic photosensitive layer.
(8) The lithographic printing plate precursor as described in the above item (5), (6) or (7), wherein the hydrophobitization precursor having a hydrophilic surface comprises composite particles containing a hydrophobic substance in the core parts and having hydrophilic property in the surface parts.
(9) The lithographic printing plate precursor as described in any of the above items (5) to (8), wherein a water-soluble protective layer is further provided on the hydrophilic photosensitive layer.